A helicopter main rotor assembly, and in particular the main rotor blades thereof, is subjected to a variety of operational forces--aerodynamic, inertial, and centrifugal. In particular, main rotor blades must be designed to accommodate various dynamic loads such a bending loads, flapwise (out-of-plane) and chordwise (in-plane) loads, axial loads (centrifugal), and torsional loads. Such dynamic loads subject the main rotor blades to varying degrees of stresses/strains.
In particular, the extreme outboard end or tip of the main rotor blade is subjected to the greatest stresses/strains due to aerodynamic forces, and concomitantly experiences the greatest structural degradation due to wear and abrasion (due to the high rotational velocity of the main rotor blade tip), during operation of the helicopter main rotor assembly. In addition, the extreme outboard end of the main rotor blade has the highest probability of experiencing structural damage as a result of striking external objects.
Accordingly, it is well known in the helicopter art to provide a sacrificial or removable tip cap at the extreme outboard end of a helicopter rotor blade. Representative examples of tip caps for helicopter rotor blades are presented in U.S. Pat. No. 4,120,610 which discloses an end tip cap secured to the nose spar of a main rotor blade; and U.S. Pat. No. 5,320,494 which describes a replaceable tip portion for a main rotor blade comprised of a composite skin bonded to an internal honeycomb core and a composite tip cap attached to the tip portion.
FIGS. 1A-1C illustrate an exemplary main rotor blade 100 for a helicopter as described in U.S. Pat. No. 5,542,820. The main rotor blade 100 comprises an inboard segment 102 configured for mounting the main rotor blade 100 to the helicopter rotor hub assembly (not shown), an intermediate segment 104, and a replaceable tip segment 106 (reference numeral 105 identifies the demarcation between the outboard end of the intermediate segment 104 and the replaceable tip segment 106). The inboard, intermediate, and tip segments 102, 104, 106 in combination define the span of the main rotor blade 100. The main rotor blade 100 has a leading edge 108 and a trailing edge 110, which in combination define the chord of the main rotor blade 100.
The inboard and intermediate regions 102, 104 of the main rotor blade 100 are fabricated from upper and lower composite skins 112, 114 that define the upper and lower aerodynamic surfaces of the blade 100, respectively, a honeycomb core 116, a spar 118, one or more counterweights 120, a leading-edge sheath 122, and adjustable trim tabs 124. The upper and lower composite skins 112, 114 are bonded to the honeycomb core 116, which functions as a low weight, structural stiffening member between the composite skins 112, 114. The spar 118 is a prefabricated component (either from composite material(s) or a metallic material such as titanium) and functions as the primary structural member of the main rotor blade 100, reacting the torsional, bending, shear, and centrifugal dynamic loads developed in the blade 100 during operation of the helicopter main rotor assembly.
One or more counterweights 120 are bonded to the spar 118 to balance the main rotor blade 100 statically and dynamically. The leading-edge sheath 122 is a prefabricated hybrid component fabricated from composite materials, e.g., woven fiberglass material embedded in a suitable resin matrix, and abrasion-resistive materials, e.g., titanium and nickel strips. The sheath 122 has a generally C-shaped configuration that defines the leading edge 108 of the main rotor blade 100 from the inboard end of the inboard segment 102 to the outboard end of the intermediate segment 104. The leading-edge sheath 122, and in particular the abrasion-resistive strips, provide abrasion protection for the leading edge 108 of the main rotor blade 100. The leading-edge sheath 122 also provides control of airfoil tolerances of the main rotor blade 100.
The replaceable tip segment 106 of the main rotor blade 106 has a defined aerodynamic configuration, e.g., rearward sweep, taper, and/or anhedral, to enhance the aerodynamic performance of the main rotor blade 100 and the main rotor assembly. The replaceable tip segment 106 comprises upper and lower composite skins bonded in combination with a honeycomb core. A recess 140 is formed in the honeycomb core of the replaceable tip segment 106, the recess 140 having dimensions slightly greater than the external dimensions of the spar 118. The replaceable tip segment 106 is attached to the spar 118 by bonding using an adhesive material such as epoxy film adhesive applied to the wall(s) defining the recess 140 and the external surface of the spar 118. Further, fasteners 145, e.g., threaded fasteners, may be used to enhance the attachment of the replaceable tip segment 106 in combination with the intermediate segment 104. Replaceable tip segments as described hereinabove are removable at the depot level, which greatly facilitates timely repair or replacement thereof.
The described embodiment of the main rotor blade 100 further includes a composite tip cap 144, e.g., graphite fibers embedded in a suitable resin matrix, that is secured, e.g., by fasteners 146, in combination with the outboard end of the replaceable tip segment 106. Alternatively, the tip cap 144 may be fabricated from a metallic material such as titanium or aluminum. The tip cap 144 is replaceable at the field level, which minimizes the time and costs associated with the replacement thereof The replaceable tip segment 106 includes an abrasion strip 150 and the composite tip cap 144 includes an abrasion strip 152. The abrasion strips 150, 152, which are typically formed from an abrasive-resistant metallic material such as nickel, in combination define the leading edge of the replaceable tip segment 106.
The Assignee of the present invention, Sikorsky Aircraft Corporation, currently provides replaceable tip cap assemblies for its UH-60 BLACK HAWK.RTM. and SH-60 SEAHAWK.RTM. (BLACK HAWK and SEAHAWK are registered trademarks of the Sikorsky Aircraft Corporation) helicopters. These replaceable tip cap assemblies include a main fairing, internal ribs, a composite closure fairing, and a nickel abrasion strip. The main fairing is fabricated from aluminum and has a defined aerodynamic configuration, i.e., upper and lower airfoils having aerodynamic surfaces, a leading edge, and a trailing edge. The internal ribs comprise a plurality of aluminum C-shaped members that are disposed within the cavity of the main fairing and secured to the upper and lower airfoils by means of aerospace fasteners. The composite closure fairing, which has an aerodynamic configuration, is fabricated from a KEVLAR.RTM.-based composite material (KEVLAR is a registered trademark of E.I. du Pont de Nemours & Co., Wilmington, Del. for an aromatic polyamide fiber of extremely high tensile strength) and bonded in combination with the main fairing using a suitable adhesive. The nickel abrasion strip is bonded in combination with the leading edges of the main and closure fairings. The aluminum tip cap assembly is bolted in combination with the outboard end of the main rotor blade.
While the aluminum tip cap assemblies described in the preceding paragraph are functionally effective for their designed purpose, such prior art tip cap assemblies do have some limitations. For example, each of these aluminum tip cap assemblies is comprised of sixteen separate components such that the manufacture and assemblage of these aluminum tip cap assemblies is both time consuming and labor intensive. In particular, the integration of the individual C-shaped rib members in combination with the main fairing is particularly time consuming and labor intensive, requiring in excess of ninety aerospace fasteners to secure the individual C-shaped rib members in combination with the main fairing. In addition, the main fairing is susceptible to fatigue cracking as a result of the high aerodynamic loading to which the tip cap assemblies are subjected during operation of the main rotor assembly such that the aluminum tip cap assemblies must be regularly inspected and replaced.
A need exists to provide a tip cap assembly for helicopter main rotor blades that is easier to manufacture and assemble, i.e., that is more reproducible, and is more cost effective that the prior art aluminum tip cap assemblies described in the preceding paragraphs. Such a tip cap assembly should have a design configuration that makes it compatible as a retrofit item for the aluminum tip cap assemblies currently utilized in UH-60 BLACK HAWK.RTM. and SH-60 SEAHAWK.RTM. helicopters.